Most signaling pathways involve labile, dynamic protein complexes that rapidly dissociate and that are therefore notoriously difficult to analyze by high resolution structural studies. In this proposal we will use protein engineering to determine the crystal structure of a dynamic signaling complex of the crucial plant stress hormone abscisic acid (ABA). ABA is an ancient signaling molecule that is found in plants, fungi, and metazoans ranging from sponges to humans. In plants, ABA is an essential hormone and the central regulator to protect plants against abiotic stresses such as drought, cold, and salinity. These stresses are major limiting factors in crop production and therefore main contributors to malnutrition due to food shortage. This is relevant to human health because malnutrition contributes to more than 50% of human disease worldwide, including cancer and infectious diseases. Understanding the detailed mechanism of ABA signaling will be critical to provide a mechanistic basis for genetic engineering of ABA pathways in plants. At the center of ABA signaling are a family of AMPK-related protein kinases that relay the ABA signal by phosphorylating transcription factors, ion channels, and second-messenger-generating enzymes. These kinases are under the control of type 2C protein phosphatases (PP2Cs) and intracellular ABA receptors. In this proposal evidence is presented for the existence of quaternary signaling complexes that contain the receptors, ABA, PP2Cs, and SnRK2s. We will use protein engineering to stabilize these complexes and make them amenable to X-ray crystallography. The structure of these complexes will provide important insight into the function of these complexes and will identify the key interacting residues for all protein-protein and protein-ABA interactions in the context of the complex. We will mutate these residues to determine the function of these interactions in biochemical and cell-based assays as well as in vivo in transgenic plants. The outcome of this project will provide a comprehensive framework for structural understanding of receptor, ABA, PP2C, and SnRK2 interactions in ABA signaling and will thus provide a mechanistic basis for modulating ABA pathways in plants to improve their water use efficiency and food production.